The invention relates in general to a defibrillator and in particular, to a defibrillator for supplying a high energy pulse at a relatively low steady current to avoid causing discomfort to a patient to whom the defibrillator pulse is supplied while providing effective atrial defibrillation.
Cardiac defibrillators are used to provide a high energy electric pulse from a high voltage capacitor to a patient's heart for the purpose of restoring normal sinus rhythm. In some cases, persons who have previously had myocardial infarctions may be prone to ventricular or atrial tachycardia, unwanted racing of one or more chambers of the heart. Such tachycardia may lead to fibrillation of the myocardium usually of the ventricle. Fibrillation is the random firing of the muscle fibers of the myocardium. The random firing prevents the uniform pulsatile contraction normally associated with pumping of the heart. When fibrillation occurs all effective pumping stops, The normal sinus rhythm must be restored within a few minutes in order to prevent damage to tissues needing large amounts of oxygen such as the brain. Failure to restore normal sinus rhythm leads to death.
Paddle-type defibrillators typically provide up to 400 joules of electrical energy through the surface of the chest and into the heart to restore the normal sinus rhythm. Typically the discharge of current leads to a "resetting" of the electrical condition of the heart cells. The cells can then depolarize in a depolarization wave traveling along the heart. The depolarization wave causes the uniform pumping contraction of the heart characteristic of normal sinus rhythm.
However, it is also known that discharging that amount of electrical energy into the body of a patient is extremely painful and can cause the patient great discomfort. Normally, however, the patient in ventricular fibrillation has already lost consciousness and does not experience pain.
It is also known that although some patients do not have problems with ventricular fibrillation, particularly the elderly, they may suffer from atrial fibrillation. Unlike ventricular fibrillation which leads to total loss of the cardiac pumping function, atrial fibrillation, while not immediately life-threatening may nevertheless be dangerous. Atrial fibrillation causes a loss in the pumping capacity of the atriums which deliver blood to the ventricles. In particular, in atrial fibrillation a portion of the blood volume, which is normally ejected from the atriums into the ventricles, will remain behind. This can lead to stagnation of the blood within the atriums and increase the risk of the formation of a thrombus or a release of an embolus. Such emboli may be pumped out of the atrium through the ventricle and out into the generalized circulation of the body. The embolus may lodge in the brain causing a cerebrovascular accident or stroke. It may cause phlebitis if it lodges in one of the limbs such as the legs. If the embolus is generated in the right atrium, it may be carried into the circulatory system of the lungs leading to a pulmonary embolism. While none of the these conditions other than stroke, are immediately life threatening, once the embolus travels and lodges the eventual damage can be extremely debilitating or even ultimately fatal.
It is presently known that in order to provide cardioversion or the restoration of normal rhythm to an atrium, this may be accomplished by the introduction of a catheter, such as a Swan-Ganz catheter, into the brachial artery of the arm. The catheter is then passed through the pulmonary artery into one of the right atrium of the heart. While the patient is normally unconscious or heavily sedated in situations where ventricular cardioversion is to take place, and time is of the essence, patients are often not so heavily sedated or are unconscious for atrial cardioversion. As a result, the normal discharge of electrical energy through the heart for atrial cardioversion can be disconcerting and in some cases painful. One reason why the atrial cardioversion current can be painful is because a typical capacitive discharge starts at a very high current and then decays exponentially with time to a low current. The large magnitude of the current at the onset tends to cause pain in and of itself. In addition, at the beginning of the exponentially-decaying current discharge the time rate of change of the current is very large. This tends to be at a rate which directly stimulates pain nerves associated with the heart and adds additional pain.
It is also known from physiological studies that the rapidly changing high onset current, which often results in the pain, is not particularly effective for atrial cardioversion. Rather, it is the total current over the exponential curve that is required to be at a minimum level in order to achieve cardioversion. Typically, for instance, five to ten joules can be used for atrial cardioversion. Thirty joules would have to be delivered to the heart for ventricular cardioversion.
Some workers in the art may have partially solved these problems by providing defibrillators having high voltage discharge capacitors that supply defibrillating current to inductors for later delivery to the patient. As discussed in U.S. Pat. No. 4,566,457 to Stemple, U.S. Pat. No. 5,249,573 to Fincke et al., U.S. Pat. No. 5,443,490 to Flugstad, U.S. Pat. No. 5,591,209 to Kroll and U.S. Pat. No. 5,607,454 to Cameron et al., the inductors are used for pulse shaping. Cameron et al. cite Anderson et al., "The Efficacy of Trapezoidal Wave Forms for Ventricular Defibrillation," Chest, 70(2):298-300 (1976) which discloses that trapezoidal waveform current pulses may be used for defibrillation. The use of triangular and trapezoidal waveforms for defibrillation is also disclosed in Schuder, J. C., Rahmoeller, G. A., and Stoeckle, H., "Transthoracic Ventricular Defibrillation with Triangular and Trapezoidal Waveforms," Circulation Research, vol. XIX, pp. 689-694, October 1966. Schuder et al. cite Schuder, J. C., Stoeckle, H., West, J. K. and Dolan, A. M., "A Very High Power Amplifier for Experimental External Defibrillation," 16th Annual Conference on Engineering in Medicine and Biology, p. 40, (1963).
A switching defibrillation system is disclosed in U.S. Pat. No. 5,222,492 to Morgan et al. The Morgan et al. system includes a capacitor coupled by a series-connected field effect transistor switch connected to an inductor. The transistor is switched multiple times during a defibrillation pulse for the purpose of providing the pulse with a sinusoidal shape. A control circuit receives feedback representative of the defibrillation pulse and controls the pulse shape by controlling the pulse widths into a gate of the field effect transistor.
What is needed then is a way of supplying a uniform current without the high onset current and high time rate of change of current produced by present atrial defibrillators.